Four-wheel drive (4WD) vehicles provide traction which is often unattainable in two-wheel drive (2WD) vehicles by delivering power to both of the vehicle's axles. Attendant with the added traction provided by four-wheel drive is the added complexity of the drive train required to control and deliver power to all four wheels as opposed to only two wheels. Four-wheel drive characteristically does not have a center differential which would maintain a fixed fraction of torque delivered to each axle, but rather maintains the same speed at each axle regardless of the torque split. The delivery of power between the front wheels and the rear wheels of the vehicle is typically handled by a transfer case mechanism which usually includes either a mechanically or electronically controlled clutch to switch between rear-wheel drive and four-wheel drive.
In all-wheel drive (AWD) systems, the front and rear axles are continually in driving engagement with the transfer case, but the system contains a center differential to maintain a fixed ratio of torque between the front and rear axle.
In addition to controlling the transfer of torque between front and rear axles of a vehicle, the transfer case may also include a planetary gear set or set of countershaft gears which provides an underdrive or overdrive ratio. Further, rather than manufacturing a new five- or six-speed transmission, an existing four-speed transmission may be used with a transfer case to provide additional speed ratios, such as for stump pulling (extreme underdrive), or in a top gear-overdrive condition.